We have received our first reports of two-spotted spider mites being found on soybean field edges in some of the drier areas of the province. One of the problems with two-spotted spider mites is that they often go unnoticed until their numbers are severe because the earlier plant damage symptoms they cause are difficult to discern. Unless closely inspected, these symptoms are mistakenly attributed to environmental causes such as drought stress.
While only a few cases have been found so far, it’s time for all of us to keep our eyes open and be aware. Here are a few quick scouting tips:
- Two-spotted spider mites prefer hot, dry weather. Infestations usually begin at field edges and move inward. Examine several areas of the field to determine the level of infestation.
- Symptoms include white speckling on upper leaf surfaces. The actual spider mites will be found on the undersides of leaves and distinctive webbing will often be present. See Figure 1 below.
- Scout for spider mites by examining individual plants. Examine leaves closely or shake plants over a white piece of paper to determine the presence of these tiny pests. You may need a magnifying glass to see them clearly.
- Spider mites are difficult to count, so economic thresholds are based on the percentage of leaves infested. Consider insecticidal control if 20% of the leaves are infested by spider mites.
- These infestations can occur following a pyrethroid insecticide application. This means that insecticides applied to control soybean aphids can trigger an increase in spider mite populations. Insecticides can also kill off predatory mite populations and other beneficial mite predators.
- Since infestation often occurs in patches, spot treatment may be effective for this pest. However, for spot treatments to be effective, infestations must be detected in the early stages and thorough field scouting must be done. Once established, the pest can spread rapidly across a field, especially if hot and dry weather predominates.
- When severe infections of spider mites occur, plants take on a very noticeable bronze to red appearance when viewed from the roadside (see Figure 2 below).
